1. Field of the Invention
The present invention relates to a touch control apparatus of an electronic musical instrument such as an electronic piano, the touch control apparatus enabling performance operators such as keys to provide satisfactory sense of touch for a player of the electronic musical instrument.
2. Description of the Related Art
On an acoustic piano, an action mechanism in which a hammer strikes strings is driven by a manipulation of a key, results in distinctive “sense of touch” being imparted to the key for a player. An electronic piano which generates musical tone signals by an electronic tone generator is also desired to reproduce sense of touch similar to that of the acoustic piano. As an art for reproducing the sense of touch, there have been two types of arts: an art for proving an action mechanism which imitates that of the acoustic piano, and an art for reproducing the sense of touch of the acoustic piano by electrically urging a key by an actuator. As for an electronic piano of the latter type, an art for controlling the actuator is referred to as “touch control (force-perception control)”.
For the touch control, the actuator for exerting a reaction force on a key is provided to increase or decrease the magnitude of the reaction force according to a current value supplied through the actuator. Because it is necessary to control the reaction force according to physical quantity relating to the operational state of the key such as the depth of depression of the key, the velocity of depression of the key or the acceleration, an electronic piano in which the touch control is performed is provided with sensors for sensing the operational state of keys. For instance, Japanese Patent Publication No. 3772491 discloses an art for obtaining position information (depth of depression of a key) by a position sensor to differentiate the position information to obtain velocity and acceleration to control reaction force on the basis of these physical quantities. In addition, Japanese Patent Publication No. 3772491 notes that differential acceleration may be used in addition to these physical quantities. However, Japanese Patent Publication No. 3772491 does not indicate any concrete scheme to utilize differential acceleration in the touch control.
Furthermore, Japanese Unexamined Patent Publication No. 2005-195619 discloses an art for directly obtaining position information and velocity information by use of a light reflective key sensor.
Furthermore, Japanese Unexamined Patent Publication No. 2006-23287 discloses an art for measuring differential acceleration of an object by use of a piezoelectric element. More specifically, the application of acceleration to an object causes deformation of a piezoelectric element provided for the object, resulting in an electric charge Q proportional to the acceleration being generated on the piezoelectric element. If the both ends of the piezoelectric element are short-circuited, a short-circuit current i which is “i=dQ/dt” passes. The short-circuit current i is proportional to differential acceleration. Therefore, the differential acceleration can be obtained by measuring the short-circuit current i.
In addition, Japanese Unexamined Patent Publication No. 2004-94160 discloses an art for laying out various kinds of electric parts (an LED and its illumination circuit) on keys without impairing the appearance of the keys.